A fuel cell is generally known as an electricity generating system which directly converts chemical energy into electric energy through an electrochemical reaction between oxygen, or air containing the oxygen, and hydrogen contained in hydrocarbon-grouped material such as methanol and natural gas. Specifically, the fuel cell has a feature that it can produce electricity generated through the electrochemical reaction between hydrogen and oxygen without combustion and provides heat as a byproduct thereof that can be used simultaneously.
Fuel cells are classified into a phosphate (or phosphoric-acid) fuel cell working at a temperature of about 150° C. to 200° C., a molten carbonate fuel cell working at a high temperature of about 600° C. to 700° C., a solid oxide fuel cell working at a high temperature of 1,000° C. or more, and a polymer electrolyte membrane fuel cell (PEMFC) and an alkali fuel cell working at a room temperature or a temperature of 100° C. or less, depending upon kinds of used electrolyte. These fuel cells work basically on the same principle, but are different from one another in kind of fuel, operating temperature, catalyst, and electrolyte.
The recently developed PEMFC has an excellent output characteristic, a low operating temperature, and a fast starting and response characteristic as compared to other fuel cells, and uses hydrogen obtained by reforming methanol, ethanol, natural gas, etc. Accordingly, the PEMFC has a wide range of applications such as a mobile power source for vehicles, a distributed power source for the home or buildings, and a small-sized power source for electronic devices.
The aforementioned PEMFC has a fuel cell main body (hereinafter, referred to as a stack), a fuel tank, and a fuel pump supplying fuel to the stack from the fuel tank to constitute a typical system. Such a fuel cell further includes a reformer for reforming the fuel to generate hydrogen gas and supplying the hydrogen gas to the stack. Therefore, in the PEMFC, the fuel stored in the fuel tank is supplied to the reformer by means of a pumping power of the fuel pump. The reformer then reforms the fuel to generate the hydrogen gas. The stack makes the hydrogen gas and oxygen to electrochemically react with each other, thereby generating electric energy.
Alternatively, such a fuel cell can employ a direct methanol fuel cell (DMFC) scheme to directly supply liquid fuel containing hydrogen to the stack and to generate electricity. The fuel cell employing the DMFC scheme does not require the reformer, unlike the PEMFC.
In the fuel cell system described above, the stack substantially generating the electricity has a stacked structure of several or several tens unit cells having a membrane-electrode assembly (MEA) and a separator (or a bipolar plate). The MEA has a structure in which an anode electrode and a cathode electrode are bonded to each other with an electrolyte membrane therebetween. The separator simultaneously performs a function of a passage through which oxygen and hydrogen gas required for the reaction of the fuel cell are supplied and a function of a conductor connecting in series the anode electrode and the cathode electrode of each MEA to each other.
Therefore, through the separator, hydrogen gas is supplied to the anode electrode and oxygen (or air containing the oxygen) is supplied to the cathode electrode. An oxidation reaction of the hydrogen gas takes place in the anode electrode and a reduction reaction of oxygen takes place in the cathode electrode. Due to movement of electrons generated at that time, electricity, heat, and water can be collectively obtained.
Here, some of the air supplied to the cathode of the MEA through the separator participates in the reaction, and the other air not participating in the reaction is discharged. The discharged air contains a large quantity of moisture generated at the time of generating the electricity in the conventional fuel cell system, when the non-reacted (or non-participating) air containing a large quantity of moisture is directly discharged into the atmosphere at a relatively low temperature, the non-reacted air contacts the atmosphere and its contained moisture is thus condensed.
As a result, the conventional fuel cell system needs to further include an additional unit for storing or recycling water generated while the non-reacted air's moisture is condensed. Therefore, it is not possible to make the size of the entire system compact, and a heat or an electric load due to the operation of the additional unit is further applied to the system to deteriorate the efficiency and performance thereof.